1. Field of the Invention
The present invention relates to load control devices for providing variable power to alternating-current (AC) loads, for example, motor loads, and more particularly, to AC fan motors. More particularly, the invention relates to a quiet variable fan speed control, which allows substantially full variability in the fan speed control.
2. Description of the Related Art
A problem with known controllers for fan motors is that some of the techniques that have been used in the past have serious disadvantages. For example, fully variable fan speed controls are known. FIG. 1A shows a prior art fully variable fan speed control 10 in which a switch 12, typically comprising a bidirectional semiconductor switch, such as a triac, is controlled by a control circuit 14 to change the phase angle at which the triac begins conducting, thereby providing variable speed control. The fan speed control 10 is coupled between an AC power source 16 and a fan motor 18. As well known to those skilled in the art, by controlling the phase angle at which the triac begins conducting (i.e., the conduction time of the triac each half-cycle of the AC power source), the amount of power delivered to the fan motor and thus the speed of the fan motor, can be controlled.
Although the prior art fan speed control 10 provides a substantially fully variable speed control, a problem with this circuit is that when a fan motor is controlled by the phase angle technique, mechanical and acoustic noises may be generated in the fan motor, which can be annoying and distracting. FIG. 1B shows the waveforms of the AC input line voltage 30A, the motor voltage 30B applied to the fan motor by the switch, and the motor current 30C through the fan motor. As can be observed from the waveforms, the motor voltage 30B has large discontinuities, and thus harmonics, which cause noise and vibration to be generated in the fan motor. FIG. 1C shows further waveforms showing the line voltage 32A and motor currents 32B, 32C through the fan motor for near low speed and near high speed operation in graphs {a} and {b}, and the line voltage 32A and motor voltages 32D, 32E across the fan motor in graphs {c} and {d} for near low speed and near high speed operation. The harmonics in the motor voltages 32D, 32E delivered to the fan motor cause significant amounts of distracting noise and vibration, and accordingly, a better solution is desirable.
FIG. 1D shows another prior art approach that provides a quiet fan speed control 20. In this approach, a plurality of semiconductor switches 21, 22, 23, for example triacs, are provided. A capacitor 24 is provided in series with switch 22 and a capacitor 25 is provided in series with switch 23. Different values of capacitance in series with the fan motor induce different fan speeds. By controlling the switches 22, 23 to selectively insert and remove the capacitors 24, 25 from the circuit, a plurality of discrete fan speeds are provided. If switch 21 of FIG. 1D is conductive, the fan motor 18 operates at full speed. If either switches 22 or 23, or combinations of these switches, are conductive, depending upon the series capacitances, the fan motor will operate at some slower speed. Accordingly, with the circuit shown in FIG. 1D, as many as four different discrete speeds can be obtained.
However, this does not allow continuous or fully or near fully variable speed control. Additional capacitors and switches can be provided to obtain more discrete speed levels, but the circuitry becomes unnecessarily complex, large, and expensive as more components are added. An example of this type of speed control is described in U.S. Pat. No. 4,992,709, issued Feb. 12, 1991, entitled SWITCHING CIRCUIT PROVIDING ADJUSTABLE CAPACITIVE SERIES VOLTAGE DROPPING CIRCUIT WITH A FRACTIONAL HORSEPOWER MOTOR, the entire disclosure of which is incorporated herein by reference.
Nevertheless, the system shown in FIG. 1D does provide a quiet fan speed control. FIG. 1E shows waveforms of the line voltage 34A, motor voltage 34B, and motor current 34C for the prior art fan speed control 20 of FIG. 1D. As can be observed, the waveforms are fairly continuous and smooth, lacking the discontinuities of the system shown in FIG. 1A. Since the switches 21, 22, 23, are either on or off, and not operated according to the phase cut technique of the fan speed control 10 of FIG. 1A, the waveforms do not exhibit discontinuities. FIG. 1F shows further waveforms of the line voltage 36A and motor currents 36B, 36C through the fan motor in graphs {a} and {b}, and line voltage 36A and motor voltages 36D, 36E across the fan motor in graphs {c} and {d} for near low and near high speed operation.
Although this prior art system provides for a quiet fan speed control, it suffers from the drawback that the speeds are not able to be controlled continuously or fully variably or even near fully variably.
Accordingly, a more satisfactory solution, which provides the advantages of quiet fan speed control as well near fully variable speed control, and even continuously variable fan speed control, is desirable.